1. Field of the Invention
The present invention relates to a CATV system. More specifically, the present invention relates to an improvement in a CATV system in which information is transmitted through a plurality of lines (coaxial cables) from a central unit and is selectively applied to a CATV converter.
2. Description of the Prior Art
A conventional CATV system comprises a central unit and terminals located at respective homes and coupled to the central unit by coaxial cables, in which information is transmitted unidirectionally or bidirectionally. Typically, one coaxial cable is used for transmission of information of a plurality of channels. However, there is a limit to the number of channels that can be transmitted by one coaxial cable due to the limit to the bandwidth of a coaxial cable. For example, assuming that information of 36 channels is to be transmitted, the maximum bandwidth frequency would become 300 MHz. Accordingly, in case where more channels are required, two or more coaxial cables are provided and means are provided for selectively coupling the coaxial cables, one at a time to a CATV converter.
FIG. 1 is a view showing a cable selecting circuit in a CATV system using a pluralitly of coaxial cables which constitutes the background of the invention. Referring to the figure, two coaxial cables 11 and 12 are provided for transmission of information of a plurality of channels using signals of given frequencies, respectively. Selecting switches 21 and 22 are provided for selecting the coaxial cable 11 or 12. The selecting switch 21 is used for selecting a coaxial cable to be connected to a CATV converter 31 and the selecting switch 22 is provided for selecting the coaxial cable to be connected to a CATV converter 32. Now in the case where selection is made from the coaxial cable 11 to the coaxial cable 12 by the switch 21, it is necessary to avoid any undesired influence to be exerted upon the other CATV converter 32. More specifically, since the frequency of the signal to be transmitted by the coaxial cable is high, when selection is made from the coaxial cable 11 to the coaxial cable 12 by the switch 21 reflection occurs with the opened contact of the switch 21 serving as an opening end, whereby a standing wave arises in the coaxial cable 11.
If and when mismatching occurring at an opened terminal of the switch 21 is large or isolation is small when the coaxial cable 12 is connected to the CATV converter 31 through the switch 21 and the coaxial cable 11 is connected to the CATV converter 32 through the switch 22, leakage could arise between channels A and B or the impedance could change. Therefore, mismatching or mixing of an image could occur at the input of the CATV converter 32, with the result that an image of a good quality could not be obtained.